A "wire" of indium only one or a few atoms wide grown on a silicon surface comprises an ideal test laboratory for studying one-dimensional (1D) metals. A new example comes from a collaboration between researchers from Yonsei University in Korea, the ALS, and the University of Oregon, who have discovered that the phase transition from metal to insulator that occurs at low temperature in indium wires on the silicon (111) surface involves not only the expected shift in the electronic structure (band-gap opening) but also a band restructuring that gives rise to an energy gap in a second band.

Three's a Crowd

For the condensed-matter physicist, the words "electronic structure" are what it's all about. Short-hand for a description of the way electrons behave in solids, liquids, molecules, and even atoms, electronic structure underlies almost all of the everyday properties of matter from structural strength to electrical conductivity. For example, metals conduct electricity because some of their electrons have access to a continuous band of energies, whereas a break or gap in the band turns the metal into an insulator.

As it happens, electronic structure is intimately tied to the atomic structure (where the atoms are). This relationship is particularly evident in so-called low-dimensional solids, such as "atomic wires" only a few metal atoms wide, so the wires provide a test bed for studying the details of the connection. When the wires are cooled, pairs of neighboring atoms along the length of the wire move a bit closer together, just enough to generate a band gap and turn the wire into an insulator. A recent case in point comes from Ahn et al., who have found that atomic wires made of metallic indium deposited on a silicon surface undergo the expected metal-to-insulator transition, but in a heretofore unobserved way. Of three bands in the metal, one disappears after donating its electrons to a second in which a gap opens up, while the third opens a band-gap in the normal way.

Low-dimensional metals have attracted much attention because of their unique electronic properties, which often lead to exotic physics, including unconventional superconductivity, charge and spin density waves, and violations of the usual rules for interactions of electrons with either other electrons or lattice vibrations and other "excitations" (non-Fermi liquid behavior). Research on 1D metals has been enriched by the synthesis of quite novel materials, of which carbon nanotubes and metallic atomic wires on surfaces are recent examples.

Atomic structure models of indium wires four rows wide running from left to right on silicon. Top: The metallic 4×1 phase. Bottom: The period-doubled 4×2 insulating phase. The arrows indicate the major displacements of the indium atoms at low temperature.

To make metallic atomic wires, researchers deposit metal atoms on insulating substrates, where the atoms form self-organized 1D atomic chain structures that are often macroscopic in length but truly nanoscopic in width, spanning just one or a few atoms. Successful recent syntheses are the formation of indium wires on a flat silicon (111) surface [the Si(111)4×1-In surface] and gold wires on a series of regularly stepped (vicinal) silicon surfaces [Au/Si(557), Au/Si(5512), and Au/Si(553)].

These atomic wires have well-defined 1D metallic electronic structures, commonly with one or more bands that are partially filled by electrons at energies up to the Fermi energy EF and with nearly ideal band dispersions (continuous dependence of electron energy on momentum k). Moreover, metal-insulator transitions occur at transition temperatures ranging from about 100 to 300 K in which the continuous bands are broken by an energy gap around EF. Since they are accompanied by lattice-period-doubling lattice distortions, the transitions are assumed to be due to the Peierls instability that is inherent in a 1D metal, especially one with a half-filled band.

The measured energy bands of indium atomic wires in the metallic state (left) and in the insulating state at 45 K (right). The shallow m2 gap is 40–80 meV, and the deep m3 gap is 160 meV. Bands are indicated by bright regions of higher photoemission intensity. Binding energy is the energy relative to the Fermi energy (EF – E).

The Korean-American collaboration made its measurements of indium wires on silicon with the soft x-ray angle-resolved photoemission endstation (Electronic Structure Factory) at Beamline 7.0.1, which provides unprecedented flexibility in collecting data in a large volume of the Brillouin zone (unit cell in momentum space) over a wide temperature range and in preparing samples in situ, two features that were crucial in the present study. The indium wires have three metallic bands, one of which is half filled while the others are less than half filled.

Schematic drawings of the triple bands in the metallic state with the nesting vector qCDW (left) and the insulating state (right). In the metallic state, m3 (black) is the half-filled band. In the insulating state, electrons in m1 (green) transfer to m2 (blue), which becomes half filled. The two energy gaps are indicated by the bands turning down before they reach the Fermi energy EF.

The team found that in the insulating state below 125 K, the expected band-gap opening of the half-filled band was accompanied by a restructuring in which the band with the smallest filling disappeared, owing to an interband interaction in which the electrons of the disappearing band transferred into the third band, which became half filled. Thus, the ground-state band structure consisted of two bands with energy gaps at the same momentum kF, a condition giving an energy advantage to the period-doubling lattice distortion associated with a charge density wave (CDW) and a Fermi surface "nesting" vector qCDW = 2kF.

At four atom rows wide, the wires are not perfectly one dimensional, so electrons can have a momentum component perpendicular to the wire (k⊥) as well as parallel (k||), and the energy band dispersion E(k||) can be different for each k⊥ (quasi-1D). Photoemission intensity maps over the two-dimensional momentum space defined by k⊥ and k|| taken at a constant energy show band contours. The wiggling contours at the Fermi energy (binding energy = 0) for the metallic phase (left) and at 0.1 eV below the Fermi energy for insulating phase (right) demonstrate the deviation of m1 (green) and m2 (blue) from the nearly ideal 1D nature exhibited by m3 (black).

The researchers believe that this first-ever observation of the band restructuring due to a strong interband interaction introduces a new gap-opening mechanism in a multiband metal. The physical origin of the direct interband charge transfer needs to be clarified but may be related to the complex interactions of the lattice with defects and with the complex multiple 1D bands with varying band fillings.

Research funding: Ministry of Science and Technology of Korea through the Creative Research Initiative Program. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.